Background: Physical activity probably protects against the risk of breast cancer after menopause, but questions remain about how rapidly and for how long this protective effect exists. Methods: We analyzed data from 59,308 postmenopausal women (2,155 incident invasive breast cancers) followed between 1993 and 2005 (8.5 years postmenopause on average) through biennial questionnaires. Multivariable Cox models included time-varying exposure data, using levels of recreational physical activity self-reported in 1993, 1997, and 2002. Results: Women with recent (within the previous 4 years) recreational physical activity levels >= 12 metabolic equivalent task-hours (MET-h)/week had a lower risk of invasive breast cancer than women with lower levels [HR, 0.90; 95% confidence interval (CI), 0.82-0.99], with no apparent dose-response relation beyond 12 MET-h/week. Associations did not vary significantly across ER/PR subtypes. Risk reductions were of the same magnitude order regardless of weight change, body mass index, waist circumference, or less recent (5-9 years earlier) physical activity levels. Among women with levels of physical activity >= 12 MET-h/week 5 to 9 years earlier, those who became less active (<12 MET-h/week) had a significantly increased risk of breast cancer compared with those who did not (HR, 1.16; 95% CI, 1.01-1.35). And, compared with the least active women at both time points, they had no significantly decreased risk of breast cancer (HR, 1.06; 95% CI, 0.87-1.29). Conclusions: Our results suggest a decrease in risk associated with recent recreational physical activity even of modest levels. Impact: Starting or maintaining physical activity after menopause may be beneficial regarding breast cancer risk. (C) 2014 AACR.